<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .box1 {
            width: 200px;
            height: 200px;
            background: pink;
            overflow: hidden;    /* 触发了BFC，形成独立盒子 */
        }
        .box2{
            width: 100px;
            height: 100px;
            background: skyblue;
            margin-top: 30px;
        }
    </style>
</head>
<body>
<div class="box1">
    <div class="box2"></div>
</div>
</body>
</html>